Chitin is a naturally abundant mucopolysaccharide which is a (1-4)-β-linked glycan composed of 2-acetamido-2-deoxy D-glucose. Application of chitin is currently limited because of its low solubility in most common organic solvents.
On the other hand, chitosan, which is the N-deacetylated derivative of chitin obtained by the partial or total alkaline deacetylation of chitin, is soluble in acidic aqueous solutions. Chitosan is composed primarily of 2-acetamido-2-deoxy D-glucose and glucosamine residues the aqueous solubility can be attributed to the protonation of the amino groups in acidic environments. It is a pH dependent cationic polysaccharide, which is known to be non-toxic, biocompatible, and biodegradable, with its degradation products being known natural metabolites. Chitosan has been evaluated in a number of medical applications including wound dressings, matrices for controlled drug delivery and as a hemostatic agent.
Chitosan is an N-deacetylated derivative of chitin which is the structural component of crustacean shells and fungal cell walls, and is obtained at a low cost from sea-food processing (Chitin: Fulfilling a Biomaterials Promise: Eugene Khor, Elsevier, Oxford, UK, 2001). The structure of chitin and chitosan are similar to cellulose where, carbon-2 of the cellulose has acetamide or amino groups, for chitin and chitosan respectively. Chitosan is an inert, hydrophilic, biocompatible, and biodegradable polymer and hence are attractive candidates for biomedical and pharmaceutical applications. Chitosan is currently investigated for various applications such as topical ocular application, as a bioadhesive polymer, penetration enhancer by opening epithelial tight-junctions and as wound dressing (Berger, et al., European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 19-34).
Various chemically modified chitosan derivatives with unique properties have been developed (Hitoshi et al., Prog. Polym. Sci. 29 (2004) 887-908). The excellent biocompatibility of chitosan, combined with its enzymatic biodegradability, makes chitosan an excellent candidate for various in vivo applications. In addition, the low cost of chitosan and its wide availability as a natural waste product, makes chitosan a very attractive polymer for wide range of applications.
Chitosan has been extensively investigated for developing hydrogels with unique properties, due to the hydrophilicity of the base polymer, and the availability of active cross-linkable groups along the polymer chain. These chitosan hydrogels were found to be excellent candidates for a variety of applications, including, controlled release of bioactive/drug molecules, as cell encapsulation matrices, and as tissue engineering scaffolds. Chemical or covalent cross-linking of chitosan making use of mainly the active amino groups along the polymer chain and ionic cross-linking making use of the cationic nature of chitosan aqueous acid solutions, have been extensively investigated for developing hydrogels for various applications.
The different chemical cross-linking agents reported for chitosan include dialdehydes such as glutaraldehyde, diethyl squarate, oxalic acid, and genipin. Apart from these small molecules, functionalized biopolymers such as poly(ethylene glycol diacrylate), oxidized cyclodextrin, telechelic-PVA, PEG dialdehydes and scleroglucan have also been investigated.
In addition to covalent cross-linking, polyelectrolyte complexes of chitosan with a wide range of anionic polymers mainly chitosan alginate system have been extensively investigated for developing drug delivery systems and porous scaffolds for tissue engineering and wound dressings.
Ionic cross-linking of chitosan has been extensively investigated, because it is a simple and mild process with no auxiliary catalyst requirements, and such a procedure has important ramifications for biomedical applications. Metallic anions such as Mo(VI) and Pt(II) have been extensively investigated for ionic cross-linking. Various anions such as sulfates, citrates, oxalates, polyphosphates, and also calcium phosphate, have been tested for the ability to form ionically cross-linked gels with chitosan. All of these ions induce the formation of pure ionic cross-linking, where the chitosan solution instantaneously becomes a gel in the presence of these ions, due to the spontaneity of the ionic reactions.
Recently a novel temperature and pH sensitive gelling system was developed using chitosan in the presence of β-glycerophosphate. In addition to β-glycerophosphate, corresponding sulfates and monosaccharide derivatives were found to exhibit the characteristic properties of β-glycerophosphate (Chenite et al., U.S. Pat. No. 6,344,488; Chenite et al., Biomaterials 21 (2000) 2155-2161; Ruel-Gariepy et al., European Journal of Pharmaceutics and Biopharmaceutics, 57 (2004) 53-63; Ruel-Gariepy et al., J Controlled Release. 82 (2002) 373-383; Molinaro et al., Biomaterials 23 (2002), 2717-2722).
Injectable in situ forming hydrogels are receiving considerable attention for a variety of biomedical applications such as sustained drug delivery, cell encapsulation and as scaffolds for tissue engineering (Tae et al., Biomaterials, 26, 5259-66, 2005). An injectable system offers several advantages including conformal matching of the implant to complex tissue shapes, delivery of large volumes of implant via minimally invasive surgery, improved patient compliance and comfort, and allows for the delivery of sensitive biomolecules and living cells because it is a gentle process. In situ forming hydrogels are potential candidates specifically for developing sustained delivery vehicles for therapeutic proteins with short half lives.
Various materials have been investigated for the development of injectable hydrogel systems based on non-degradable synthetic polymers such as poloxamers, N-isopropylacrylamide and a variety of degradable natural polymers (Hatefi and Amsden, J. Control Rel., 80:9-28, 2002). One of the most extensively investigated natural polymers for hydrogel development is chitosan. Chitosan is an N-acetylated derivative of the natural polymer Chitin. Chitin is the structural component of crustacean shells and fungal cell walls and is the second most abundant natural polymer. Due to the excellent biocompatibility and enzymatic degradability of chitosan, hydrogels based on chitosan have been found to be excellent candidates for a variety of medical and pharmaceutical applications (Berger et al., Eur. J. Pharm. BioPharm., 57:19-34, 2004).
Different types of cross-linking agents have been investigated for developing chitosan hydrogels. These include chemical cross-linking using various aldehydes, ionic cross-linking using various anions, and polyelectrolyte complexes using anionic polymers. Recently much research has gone into developing stimuli sensitive injectable systems based on chitosan. It has been found that addition of certain polyol counterionic monohead salts such as β-glycerophosphate can lead to the development of temperature and pH sensitive gelling systems (Chenite et al, Biomaterials, 21:2155-61, 2000). Grafting poly(ethylene glycol) of appropriate molecular weight to chitosan has been shown to act as a thermogelling system (Bhattarai et al., J. Control Rel. 103:609-624).
There is a long felt need in the art for compositions and methods to prepare and use biocompatible chitosan. The present invention satisfies this need.